1. Field of the Invention
The present invention relates to methods and apparatus for producing low pressure plasmas suitable for use in semiconductor device processing. More particularly, the present invention concerns methods and apparatus for igniting low pressure plasma discharges.
2. Description of Related Art
A variety of semiconductor device processing techniques include exposure of a semiconductor wafer to some form of plasma. Semiconductor wafer etching, passivation, and deposition processes, for example, typically involve exposing the semiconductor wafer to a plasma formed from some appropriate process gas. The plasma is normally formed at low pressures by the transfer of kinetic energy from excited electrons within the plasma to the molecules of the process gas through individual collisions between the electrons and the gas molecules. Energy is usually imparted to the electrons from a power source by capacitive coupling, inductive coupling, or both. Capacitive coupling relies on the generation of an electric field within the plasma to accelerate the electrons, while inductive coupling additionally involves the use of an oscillatory magnetic field to excite the electrons. Inductive coupling also restricts the linear motion of the electrons with the resultant effect of improving plasma containment and density.
Typically processing of a semiconductor wafer with a plasma is performed in a partially evacuated chamber. Considerable effort has been directed to enhancing the transfer of energy from electrons within the plasma to the molecules of the process gas, while minimizing the effect of the electrons and the ions on the semiconductor wafer itself. Once common form of capacitive energy transfer, for example, involves the application of a radio frequency (RF) electric field between two parallel plates disposed within the plasma chamber and oriented parallel to the wafer. This approach, however, suffers from a number of disadvantages. At low operating pressures a substantial portion of the energy imparted by the electric field to the plasma is typically dissipated through collisions with the structure of the plasma chamber, or collisions with the silicon wafer itself. Because of these collisions with the wafer, limitations are imposed on the operating conditions of the process to avoid harm to the semiconductor. The semiconductor wafer may also be adversely affected by increases in thermal temperature, a further consequence of electron and ion collisions with the wafer. Thus plasma processing techniques relying on the application of a RF electric field to a plasma are usually limited to certain pressure ranges, commonly above about 0.01 torr, and are typically employed only in combination with the application of a very high frequency RF electric field.
To increase the efficiency of plasma generation by capacitive coupling, microwave resonance type plasma generations have been developed, employing ultra high cavity frequencies on the order of 2.45 gigahertz. These high frequencies increase the likelihood of traferring electron energy to the process gas molecules, rather than the plasma chamber or the semiconductor wafer, by decreasing the oscillatory path of the electrons. Another approach, termed electron cyclotron resonance, employs a controlled magnetic field to induce a circular electron flow path within the process gas. Still another approach to capacitively coupled plasma generation employs a large and uniform magnetic field oriented parallel to the surface of a semiconductor wafer in combination with a high frequency RF field oriented perpendicular to the semiconductor wafer. The combination of these two fields imparts a cycloidal flow path to the electrons, thus increasing the probable distance travelled by the electrons before collision with the chamber or the wafer.
As noted above, inductively coupled plasma generating systems primarily employ oscillating magnetic and fields to impart energy to electrons within a plasma chamber. Inductively coupled systems are typically configured much like a transformer, with one coil of the tansformer located outside the plasma chamber and with an electron stream within the plasma forming the second coil of the transformer as the electrons travel in a generally closed path through the process gas. One form of conventional inductively coupled plasma generator is disclosed in U.S. Pat. No. 4,948,458 to John Ogle ("the '458 system"). This generator normally provides a plasma deemed suitable for a variety of semiconductor wafer processing methodologies. The '458 system includes a chamber with a dielectric window and a planar coil disposed outside the chamber adjacent the window. A resonant RF current is induced in the planar coil, which in turn produces a magnetic field within the chamber to create a stream of circulating electrons, and thus a plasma, in a plane parallel to the exterior coil. While generally considered suitable for the plasma treatment of semiconductors over a relatively broad range of operating pressures, initial ignition of a plasma in this system may be difficult at pressures in the low millitorr range. In applications where a lower process pressure is desired, the plasma must first be initiated at a higher pressure and then reduced to a lower operating pressure. There still exists a need, however, for a plasma generator capable of creating highly uniform plasmas over a broad range of operating pressures. The present invention fulfills this need.